This invention relates to an improved elapsed time indicator. More particularly, it relates to an elapsed time indicator which is substantially unaffected by normal temperature and humidity variations.
It has been found that there is a need for an inexpensive but accurate time indicator which provided a clear and measurable indication of the passage of a predetermined time. The ideal device should be substantially independent of physical changes in its surroundings, such as changes in temperature and humidity. A particular need for such a device has arisen for use with intravenous tubing particularly in hospital applications.
In order to prevent infection, intravenous tubing in use by a patient should be replaced every 24 hours. The procedure now, in most cases, is for the nurse to check her watch and record the time that the fluid first begins to flow through the intravenous tubing which is attached to the patient. She then tries to remember when the 24 hour period has elapsed so that she may change the tubing. Another mechanism which has been tried is the use of four differently colored dots which are attached to the tubing and which are individually torn off as the nurse makes her rounds every six hours, thus when the last dot is torn off, the nurse would know that 24 hours has elapsed. It has been found that, due to human error, these particular types of methods of providing an indication of when the tubing is to be replaced have not worked.
There are various active chemical indicators, which provide visible information as to the elapse of time. One such indicator is set forth in U.S. Pat. No. 3,620,677, assigned to the Miles Laboratory, Inc. However, the Miles patent is more useful as a time-temperature indicator, that is, a device which integrates both time and temperature since Miles utilizes a chemical reaction in the indication process which is affected by changes in temperature. Other devices have been suggested which would act as a time indicator. One such device is set forth in U.S. Pat. No. 4,137,049, issued Jan. 30, 1979, entitled A Device For Use as an Elapsed Time Indicator or Time Temperature Indicator, and assigned to Akzona Incorporated, assignee of the present invention. While this device functioned quite well as a time indicator for a certain range of temperatures, it was found that it was affected by changes in relative humidity. Thus, it is desirable to provide an improved time indicator which overcomes the deficiencies of the prior art.